. Scientific Frontline: Hot on the trail of the causes of rapid ice sheet in­stabil­it­ies in cli­mate his­tory

Wednesday, July 27, 2022

Hot on the trail of the causes of rapid ice sheet in­stabil­it­ies in cli­mate his­tory

The re­search ves­sel MARIA S. MERIAN leav­ing the har­bor of St. John’s (Canada). As a par­ti­cipant on Ex­ped­i­tion MSM 39 (2014), Lars Max, along with other re­search­ers, ob­tained the sample ma­ter­ial for this study.
Credit: MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences, Uni­versity of Bre­men; D. Kieke

Extreme cooling events during the last glacial, known as Heinrich Events in the North Atlantic, are a good example of how local processes change the global climate. While the impacts of Heinrich Events on the global glacial environment are well-documented in the scientific literature, their causes are still unclear. In a new study, researchers from Bremen, Kiel, Köln and São Paulo (Brazil) have now shown that an accumulation of heat in the deeper Labrador Sea caused instabilities in the Laurentide Ice Sheet, which covered much of North America at the time. The Heinrich Events were triggered as a result. The researchers demonstrated this by reconstructing past temperatures and salinities in the North Atlantic. Their results have now been published in Nature Communications.

Hein­rich Events or, more ac­cur­ately, Hein­rich Lay­ers, are re­cur­rent con­spicu­ous sed­i­ment lay­ers, usu­ally ten to 15 cen­ti­meters thick, with very coarse rock com­pon­ents that in­ter­rupt the oth­er­wise fine-grained oceanic de­pos­its in the North At­lantic. Dis­covered and de­scribed for the first time in the 1980s by geo­lo­gist Hart­mut Hein­rich, U.S. geo­chem­ist Wally Broecker later of­fi­cially named them Hein­rich Lay­ers, which has be­come a stand­ard term in pa­leocean­o­graphy.

The pres­ence of Hein­rich Lay­ers has been es­tab­lished throughout the North At­lantic, from off Ice­land, south­ward to a line run­ning from New York to North Africa. Such coarse rock debris could only have been trans­por­ted such a great dis­tance from its point of ori­gin in the Hud­son Bay by ice­bergs.

Deep sea sediment core with coarse ice-transported lithogenic components (Heinrich layer).
Credit: Lars Max

“The ac­tual sig­ni­fic­ance of these Hein­rich Events, however, lies in the fact that, along with the melt­ing phase and re­lease of ice­bergs, large quant­it­ies of fresh wa­ter were in­tro­duced into the North At­lantic,” says Lars Max, pa­leocean­o­grapher at MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences at the Uni­versity of Bre­men and first au­thor of the study. As part of their work, he and his co-au­thors re­con­fig­ure the in­ter­re­la­tion­ships among Hein­rich lay­ers, fresh­wa­ter sup­ply, and changes in the ocean cir­cu­la­tion. A thin fresh­wa­ter lens ly­ing atop mil­lions of cu­bic kilo­met­ers of wa­ter dur­ing the Hein­rich events is presently con­sidered to be the cause of the dis­rup­tion of the At­lantic Me­ri­di­onal Over­turn­ing Cir­cu­la­tion (AMOC), or its com­plete shut­down, with pro­found re­gional and global cli­matic con­sequences. The AMOC is just one seg­ment of the global con­veyor belt of ocean cur­rents that is driven by tem­per­at­ure and sa­lin­ity and plays a sig­ni­fic­ant role in the cli­mate sys­tem.

“Ori­gin­ally the dis­rup­tion was con­sidered to be the res­ult of in­ternal in­stabil­it­ies of the ice sheet it­self. Our study, however, provides evid­ence that changes in the ocean had a destabil­iz­ing im­pact on the ice sheet on the North Amer­ican con­tin­ent,” says Lars Max. The study of a sed­i­ment core ob­tained by the re­search ves­sel MARIA S. MERIAN at the out­let to the Lab­rador Sea in the North At­lantic provides the first solid evid­ence of re­cur­rent, massive ac­cu­mu­la­tions of ocean heat in the deeper lay­ers of the sub­polar North At­lantic. This fa­cil­it­ated the melt­ing of the po­lar ice sheets from be­low.

“Us­ing trace-ele­ment and iso­topic ana­lyt­ical meth­ods, we were, in fact, able to re­con­struct tem­per­at­ure and sa­lin­ity in­creases at around 150 meters of wa­ter depth that al­ways sys­tem­at­ic­ally pre­ceded the Hein­rich Events in time, and that cor­res­pon­ded to times of an already weakened At­lantic Me­ri­di­onal Over­turn­ing Cir­cu­la­tion,” ex­plains Dirk Nürn­berg of the GEO­MAR Helm­holtz Centre for Ocean Re­search in Kiel, who is re­spons­ible for the labor­at­ory ana­lyses.

This sug­gests that changes in ocean cir­cu­la­tion triggered the ice-sheet in­stabil­it­ies. A con­tinu­ous warm­ing of the ocean at this depth was crit­ical for destabil­iz­ing the ice shelf from be­low, and even­tu­ally led to the ac­cel­er­ated shed­ding of ice­bergs – the Hein­rich Events.

Planktonic microfossils such as the species Neogloboquadrina pachyderma sinistral carry the isotope geochemical information used to perform oceanographic and climatic reconstructions.
Credit: Antonov, Public domain, via Wikimedia Commons

Un­der­stand­ing the pro­cesses from Earth’s his­tory also en­ables us to bet­ter pre­dict changes that can be ex­pec­ted to ac­com­pany the cur­rent global warm­ing. “If the over­turn­ing cir­cu­la­tion should weaken in the fu­ture due to an­thro­po­genic cli­mate change,” sug­gests Chris­ti­ano Chiessi of the Uni­versity of São Paulo, “we would ex­pect an ac­cel­er­ated warm­ing of the deeper sub­polar North At­lantic that could neg­at­ively im­pact both the sta­bil­ity of the present-day Arc­tic gla­ciers and the fresh­wa­ter budget of the North At­lantic.”

The latest In­ter­gov­ern­mental Panel on Cli­mate Change (IPCC) As­sess­ment Re­port (2021) con­cludes that, with con­tin­ued warm­ing of the cli­mate, there could be a weak­en­ing of the over­turn­ing cir­cu­la­tion in the At­lantic Ocean within this cen­tury. In­tens­i­fied warm­ing of the deeper sub­polar North At­lantic and more rapid melt­ing of the Arc­tic gla­cial masses could also have the res­ult of fur­ther ac­cel­er­at­ing the global rise in sea level. As Lars Max also points out, however, we can ex­pect that the sta­bil­ity of the Ant­arc­tic Ice Sheet will play a sig­ni­fic­ant role in the course of sea-level rise. Fur­ther stud­ies are cru­cially needed in or­der to bet­ter pre­dict to what ex­tent the fu­ture de­cel­er­a­tion of over­turn­ing cir­cu­la­tion and pos­sible warm­ing of the deeper ocean could have on the fu­ture sta­bil­ity of the Ant­arc­tic Ice Sheet.

Source/Credit: MARUM / Cen­ter for Mar­ine En­vir­on­mental Sci­ences, Uni­versity of Bre­men


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